1. Field of the Invention
The present invention relates to an organometallic complex that is capable of converting an excited triplet state into luminescence, a light-emitting element using the organometallic complex, and a light-emitting device using the light-emitting element.
2. Description of the Related Art
A light-emitting element using an organic compound is an element in which a layer including an organic compound or an organic compound film emits light by applying an electric field. The emission mechanism is said to be as follows: when a voltage is applied between electrodes with an organic compound film interposed therebetween, an electron injected from a cathode and a hole injected from an anode are recombined in the organic compound film to form a molecular exciton, and energy is released to emit light when the molecular exciton returns to the ground state.
In such a light-emitting element, generally, an organic compound film is formed to be a thin film of less than 1 μm. In addition, since such a light-emitting element is a self-luminous element in which the organic compound itself emits light, a backlight as used for a conventional liquid crystal display is not necessary. Therefore, such a light-emitting element has a great advantage of being able to be manufactured to be thin and lightweight. In addition, for example, in an organic compound film on the order of 100 to 200 nm, the time from injection of carriers to recombination is approximately several tens of nanoseconds in consideration of the carrier mobility of the organic compound film, and light gets to be emitted approximately within microseconds even when the process from the recombination of the carriers to light emission is included. Therefore, it is also one of features that the response speed is quite fast. Further, since such a light-emitting element is a carrier-injection type light-emitting element, driving by a direct voltage is possible, and noise is not easily generated. As for the driving voltage, a sufficient luminance of 100 cd/m2 is achieved at 5.5 V when an organic compound film is a uniform ultra thin film approximately 100 nm in thickness, an electrode material is selected so as to reduce the carrier injection barrier for the organic compound film, and further, a hetero structure (a two-layer structure here) is introduced (for example, Reference 1: C. W. Tang, et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
In addition to such element characteristics such as slimness, lightweight, high-speed response, and direct-current low-voltage driving, it can be also said to be one of great advantages that the luminescent color of a light-emitting element using an organic compound is rich in variation, and the factor is the variety of organic compound themselves. Namely, the flexibility of being able to develop materials for various luminescent colors by molecular design (for example, introduction of a substituent) or the like produces richness of colors. It can be said that the biggest application field of a light-emitting element utilizing this richness of colors is a full-color flat-panel display because there are a lot of organic compounds capable of emitting light's primary colors of red, green, and blue, and thus, full-color images can be achieved easily by patterning of the organic compounds.
It can be said that the above-described element characteristics such as slimness, lightweight, high-speed response, and direct-current low-voltage driving are also appropriate characteristics for a flat-panel display. However, in recent years, the use of not fluorescent materials but phosphorescent materials has been tried as an attempt to further improve a luminous efficiency. In a light-emitting element using an organic compound, luminescence is produced when a molecular exciton returns to the ground state, where the luminescence can be luminescence (fluorescence) from an excited singlet state (S*) or luminescence (phosphorescence) from an excited triplet state (T*). When a fluorescent material is used, only luminescence (fluorescence) from S* contributes.
However, it is commonly believed that the statistical generation ratio between S* and T* of a light-emitting element is S*:T*=1:3 (for example, Reference 2: Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, p. 31 (1993)). Accordingly, in the case of a light-emitting element using a fluorescent material, the theoretical limit of the internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% on the ground of S*:T*=1:3. In other words, in the case of a light-emitting element using a fluorescent material, at least 75% of injected carriers are wasted uselessly.
Conversely, it is believed that the luminous efficiency is improved (simply, 3 to 4 times) if luminescence from T*, that is, phosphorescence can be used. However, in the case of a commonly used organic material, luminescence (phosphorescence) from T* is not observed at room temperature, and normally, only luminescence (fluorescence) from S* is observed. In reality, in recent years, light-emitting elements in which energy (hereinafter, referred to as “triplet excitation energy”) that is emitted on returning from T* to a ground state can be converted into luminescence have been released one after another, and the high luminous efficiency has attracted attentions (for example, Reference 3: J. Duan et al. Advanced Materials, 2003, 15, No. 3, Feb. 5 pp. 224-228).